The present invention relates to a navigation system mounted on vehicles such as automobiles, aircrafts and vessels, and more particularly to a system for detecting an altitude in the navigation system.
In a navigation system the present position of a vehicle is detected by such a position detecting means as a Global Positioning System (GPS). A receiver mounted on the vehicle receives radio waves transmitted from a plurality of GPS satellites to calculate the present position. A mark representing the position is indicated on a map shown on a display.
The GPS generally uses three or more GPS satellites to determine a two-dimensional position of the vehicle. That is the latitude and longitude are calculated based on position data of each satellite and a pseudo-distance between the vehicle and each satellite including a time offset caused by a time lag of the receiver.
In another example of the GPS, radio waves are transmitted from more than four satellites, thereby enabling to detect the three-dimensional position of the vehicle. Namely, the altitude of the vehicle is further calculated in addition to the latitude and the longitude.
However, due to circumferential conditions such as mountains and buildings, the radio waves from the satellites are often blocked, thereby rendering the three-dimensional positioning impossible. The conventional GPS accordingly calculates the altitude only when radio waves from more than four satellites are received. When radio waves from only three satellites are received, the two-dimensional position is calculated, setting the known altitude calculated at the last three-dimensional positioning as the current altitude.
However, the known altitude may deviate from the actual altitude, especially when the vehicle is driven in a mountainous region where the altitude largely fluctuates. The difference between the actual altitude and the set altitude increases with a lapse of time, thereby causing error in the calculated position of the vehicle.
Alternatively, an external memory, such as a CD-ROM storing a corresponding altitude obtained from a map or obtained by a manometer for detecting altitude dependent on ambient pressure of the vehicle, is provided on the vehicle for the GPS to determine the actual altitude. Upon the three-satellite detection, an altitude derived from the memory is used to calculate the position of the vehicle. The difference between the altitude calculated at the last four-satellite detection and the actual altitude may be so great that a mark indicating the position of the vehicle on the display abruptly jumps to another position.
FIG. 12 shows an example of altitude variation. The line AL shows an actual variation of altitude, a point A is a last detection position of the four-satellite detection, and a point B is a resumed detection position by the four-satellite detection. Therefore, the detection between points A and B is performed by the three-satellite detection. The altitude Hp at the point A is obtained by the memory, and the altitude H.sub.G at a point A' is obtained by the four-satellite detection. The altitude Hc at the point B is obtained by a memory, and the altitude Hc' at a point B' is obtained by the four-satellite detection. There is a large deviation from the actual altitude Hc.